Ferritic stainless steel

ABSTRACT

A hot rolled ferritic stainless steel having up to 0.07% carbon, 10-12.5% chromium, traces to 0.75% silicon, traces to 3% manganese, traces to 2% copper, 0.5 to 1.5% nickel, 0.1 to 0.5% titanium and which may contain as small alloying additions aluminum, vanadium, zirconium, columbium or combinations thereof. The elements of the steel are also balanced with respect to each other so that it is capable of being softened to RB80 hardness or lower in less than about 24 hours of subcritical box annealing and develops at least 50% austenite during hot rolling so as to exhibit a charpy V-notch impact strength of at least 20 ft.-lb. at 0* F.

United States Patent [451 Mar. 21, 1972 Aggen [54] FERRITIC STAINLESS STEEL [72] Inventor: George Aggen, Sarver, Pa.

[73] Assignee: Allegheny Ludlum Steel Corporation, Pittsburgh, Pa.

[22] Filed: Jan. 31, 1969 [21] Appl. No.: 795,740

[52] U.S.Cl. ..75/128 T, 75/125,75/128 T, 75/128 V [51] Int. Cl ..C22c 39/20 [58] Field of Search ..75/126 R, 125

[56] References Cited UNITED STATES PATENTS 2,056,765 10/1936 Becket ..75/126 R 2,590,835 4/1952 Kirkby..

2,848,323 8/1958 Harris ..75/126 R Primary ExaminerHyland Bizot Attorney-Richard A. Speer and Vincent G. Gioia [5 7] ABSTRACT A hot rolled ferritic stainless steel having up to 0.07% carbon, 1012.5% chromium, traces to 0.75% silicon, traces to 3% manganese, traces to 2% copper, 0.5 to 1.5% nickel, 0.1 to 0.5% titanium and which may contain as small alloying additions aluminum, vanadium, zirconium, columbium or combinations thereof. The elements of the steel are also balanced with respect to each other so that it is capable of being softened to R 80 hardness or lower in less than about 24 hours of subcritical box annealing and develops at least 50% austenite during hot rolling so as to exhibit a charpy V-notch impact strength of at least 20 ft.-lb. at 0 F.

2 Claims, No Drawings FERRITIC STAINLESS STEEL A few years ago a ferritic stainless steel was developed which possesses the attractive features of low cost, good formability, good weldability and adequate corrosion rethe limits on carbon and silicon are comparable to that for the MF-l type steels and for the same reasons. In the MF1 steels titanium is added to combine with carbon to result in a low matrix carbon content so as to improve ductility and to help sistance for many applications. However, this alloy, which is assure the ferritic character of the steel. in the present invenknown commercially as MF-l, (described in US. Pat. No. tion it is desired to improve toughness and accordingly the ele- 3,250,61 1) has a relatively low strength (about 40 k.s.i. yield ments must be balanced to achieve this result. This can be acstrength) unless cold worked and a high impact transition temcomplished by additions of up to about 3% manganese, up to perature, i.e., about 70 F. At 0 F. this alloy generally has a about 2% copper or with small but effective amounts of nickel charpy V-notch impact strength of about 4 ft.-lb. The high imin an amount of from 0.5 to 1.5%. It can also be accomplished pact transition temperature in particular detracts from its useby substituting aluminum, vanadium or columbium or comfulness for certain structural applications. It is desirable, binations of these elements for titanium. Substitutions are therefore, to improve the strength and toughness of MF-l preferably at least about 0.05%. Additions of manganese or type steels without adversely affecting the steels other beneficopper or nickel or combinations thereof decidedly lower the cial properties and without unduly increasing its cost. chromium equivalency of resulting alloys. When an alloy con- The present invention provides an improved ferritic staintains sufficient amounts of these elements the chromium less steel having superior toughness and improved strength equivalency will be below 10 insuring the formation of at least while maintaining adequate formability. Formability is conabout 50% austenite at hot rolling temperatures within the veniently measured by the hardness of the alloy and ferritic a ge of h F and the attainment of 8 stainless steels in accordance with the invention are capable of toughness. Nickel content. however, must not eXCeed about being softened to a R,, hardness of 80 (or lower) in less than 1.5% or the alloy will not possess adequate formability. It will about 24 hours by subcritical box annealing, thus avoiding the not be capable of being softened to an R hardness of 80 or necessity for unduly costly heat treatments. The strength level lower in less than about 24 hours of subcritical box annealing. of the improved ferritic stainless steel in accordance with the 25 Carbon and nitrogen can serve in the same manner as maninvention is not obtained by cold working and thus is not ganese, copper and nickel but are undesirable since they result limited to certain product forms Such as hose which woul in the formation of a hard and brittle martensite which can not h the effects of cold Working destroyed y Certain pose processing difficulties and welding problems. Aluminum, fabflCahOh P -g-. u gh Wlth the vanadium and columbium are weaker ferrite formers than invent on there rs provided a ferritic stainless steel consisting titanium or zirconium which behaves Similarly to titanium and essentially of P Carbon, Prefarably less than 097% therefore do not hinder the formation of austenite at hot Carbohv P to 075% 511mm Pf y to 045%, P to 3% rolling temperatures to the same degree as does titanium or manganese, F 15% h i up 2% S PP P to zirconium. They are, however, effective carbide or nitride forofany ofmflmumqalummumr P zn'comum 1 mers as are titanium and zirconium and may be useful in to elements bemg balanced to Pmvde minimizing the formation of hard brittle martensite. Hence, 9 chrommm equ.walency 1 of not greater than 10 accord through balancing of elements the present invention provides mg toihe followmg 9 2 5 7 A1 many of the advantages of the MF-l type steels including rela. g g fg y gzv 2 C+N tively low cost and excellent formability plus the additional i c 2:; o a advantage of materially improved toughness, i.e., an improve- 0 1) a n u 40 ment in the impact transition temperature from about 70 F. so as to develop at least about 50% austenite (by volume) at f MF 1 t b l b oo hot rolling temperatures in the range of 1 600-2 200 F and or o e ow a on o Corrosion tests have proven that the steels in accordance attain an impact strength of at least 20 ft.-lb. at 0 F.;sa1d steel b N f ftenin to R 80 or lowe b Subcrmcal box with the present invention have corrosion resistance roughly emg e 0 so g B r y comparable to the MF-l type steels. The zirconium containannealmgm less than about 24 hours. t h v Somewhat im mved As described above, it is necessary that the elements in the z z gg szi g z j salt S ta tgsts steel in accordance with the invention be controlled and slsjfmqi' O 0 p balanced in order to achieve the desired combination of proh fcfnowmg exampkfs W111 Inmate the pram i perties characteristic of the invention. Thus, for example, the vemlon (1th Presently Preferred Chromium must be present in a quantity f at least 0 to Also described below are comparative results with steels outsure satisfactory corrosion resistance and addition of this ele- 4 1 9 9! f i above ment beyond about 12.5% is an unnecessary expense, as the Fifty pound heats were melted in an argon atmosphere. The additional corrosion resistance provided is not required for argon was used to simulate the cleanliness and gas contents many applications. The chromium level in the present invenobtained during normal arc-furnace melting of commercial tion is similar to that of MF-l type steels. In the same manner beats. An analysis of the heats is found below in Table I.

TABLE I Heat No. C Mn P S 31 Cr N1 N Ti Al V Cb Cu Zr RV-1787 .043 .50 .015 .011 .39 RV-1788 .046 .54 .013 .012 .42 RV-1770- .044 .49 .008 .010 .50 RV178 .045 .54 .013 .011 .39 RV-1781- .050 .50 .013 .011 .36 RV-1768. .048 .009 .009 .5 .010 .010 .5 .012 .009 .42 013 010 42 .013 .009 .41 015 010 42 008 014 22 .008 .018 .18 .009 .018 .17 .009 .011 .18 010 012 17 .012 .013 .18 .010 .010 .12 .011 .014 .15 010 015 10 .010 .018 .10 .010 .018 .10

1 Estimated average depending on whether Tl or A1 or T1 and Al present.

TABLE III [Charpy V-Noteh Impact, tt.-1bs., at indicated temperatures] Degrees F.

Heat No. 100 50 ---26 32 78-80 125 170 212 RV-1787 RV-1788 00 37 RV-1770. 196/218 RV-1780. 218/227 1 Average of two values 67 and 13. All the heats except for RV-1787, lav-1771. it is noted t hat all the alloys attained a charpy V-notch impact RV-1783, RV1786, RV-1784 and KA are within the scope of the present invention. Heat KA30 is outside the invention because of high nickel, over about 1.5%, and the others are outside the invention because of chromium equivalencies in excess of 10. The chromium equivalency for all the heats is given below in Table [1. Attention is directed to heat RV-l787 which is a typical MF-l heat.

TABLE 11 All the heats found in Table 1 were hot worked and annealed. More particularly the RV series were pressed to 1%- inch squares from 2,200 F., hot rolled to %-inch bars from 1,950 F. and annealed for one-half hour at 1,500 F. The KA series were pressed to slabs 2 X5 inches from 2,350 F., hot rolled from 2,225 F. to 16-inch thickness, reheated to l,525 F. and hot rolled at 1,525 F. in one pass to a 14-inch thickness. KA samples belonging to the 1(A* series, see Table 111, were additionally annealed at 1200 F. for 1 hour and KA samples belonging to the KA** series were additionally annealed at 1,400 F. for 1 hour. The KG series samples were forged to l'hinch from 2,000 F., reheated and forged to A-inch bars with a finishing temperature below 1,500 F. and annealed. The KG samples belonging to the KG* series were annealed for one-half hour at 1,500 F. and air cooled while KG samples belonging to the KG** series were annealed for 1 hour at 1,350 F. and air cooled. The KG sample belonging to the KG*** series was annealed by holding for 1 hour at 1,550 F., furnace cooling to 1.200 F. and air cooling to room temperature.

Table [11 below summarizes the results of charpy V-notch impact tests carried out over a range of temperatures for the samples whose compositions are given in Table 1.

strength of at least 20 ft-lb. at 0 F. except for those singled out earlier for having chromium equivalencies in excess of 10. These are the alloys which do not develop at least about 50% austenite (by volume) at hot rolling temperatures in the range of 1,600-2,200 F. For example, heat RV-1783 containing in excess of 0.8% zirconium is fully ferritic and hence exhibits a high impact transition temperature.

As stated earlier the alloys in accordance with the invention are capable of being softened to an R hardness of in less than about 24 hours by subcritical box annealing. The ability to be softened is an important factor for good formability. To insure satisfactory formability the nickel content must be maintained below about 1.5%. In order to show the criticality of nickel, 50 pound heats of KAI l, 1(A26, KA29 and KA30, whose compositions can be found in Table I, were processed by pressing to 2X5-inch slabs from 2350 F., hot rolling to 0.150 inch from 2,225 F., annealing for 15 minutes at 1,500 F., cold rolling to 0.090 inch and final annealing for 5 minutes. Final annealing temperatures are shown in Table IV along with the members hardness values. It is noted that member KA30 which contains more than 1.5% nickel, could not be softened sufficiently so as to attain good formability, even with treatments known to provide maximum softness. Members KAl 1, KA26 and KA29 showed that alloys containing from about 0.5% to about 1.5% nickel, up to 0.07% carbon and from 0.1 to 0.5% titanium could attain a charpy V-notch impact strength of at least 20 ft.-lb. at 0 F. and could be softened. Of course, alloys with nickel levels in excess of residual amounts and less than 0.5%, i.e., about 0.35%, can also be softened.

Four additional heat treatments were performed with KA30 to see if it could be softened below R 80. Found below in Table V are the heat treatments along with the respective hardness values obtained.

TABLE V it is noted for contrast that KA26 (1% Ni) given heat treatmerit B and D exhibits hardness of R,,67.7 and R 715.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. it is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

I claim:

1. A hot rolled ferritic stainless steel consisting essentially of, in weight percent, up to 0.07% carbon, lO-l2.5% chromium, up to 0.75% silicon, up to 3% manganese, 0.5 to l.5% nickel, 0.1 to 0.5% titanium, up to 0.8% aluminum, up to 1.0% columbium, up to 0.8% zirconium, up to 0.8% vanadium, up to 2% copper, balance essentially iron, said elements being balanced to provide a chromium equivalency (Cr of not greater than 10 according to the following equation:

so as to be capable of developing at least 50% austenite (by volume) at hot rolling temperatures in the range of l,6002 ,200 F. and attaining a charpy V-notch impact strength of at least 20 ft.-lb. at 0 F., said steel being capable of softening to R by subcritical box annealing in less than about 24 hours.

2. A ferritic stainless steel according to claim 1 having 0.2 to 0.6% silicon. 

2. A ferritic stainless steel according to claim 1 having 0.2 to 0.6% silicon. 